Hitherto, preparation of solutions has been carried out in a tank system. In the tank system, prescribed quantities of dissolving solution (solvent) and powder are introduced into a solution tank and stirred by a stirring pump or a stirring blade and mixed to form a solution. The prepared solution is transferred to a point of use by a delivery pump. At this time, the level of the surface of the solution in the solution tank is lowered, and a negative pressure is generated in the solution tank and thus outside air is introduced into the solution tank. The introduction of air occurs in the tank system because the solution tank is generally opened to the air to prevent breakage of the solution tank itself by a negative pressure generated therein. Therefore, in many cases, an air filter is provided at a portion opened to the air to prevent bacteria or the like contained in the outside air from entering. The use of an air filter results in problems and a high cost for replacing the air filter on a regular basis. As a matter of fact, there are cases where a filter that prevents only dust is used, or even no filter is used considering the problems and cost. In addition, in a tank system, when an attempt is made to prepare a large quantity of solution at a time, a large solution tank is necessary, thereby disadvantageously increasing the size of the apparatus itself. Moreover, since many stirring pumps and delivery pumps are necessary, the operating noise may disadvantageously be too loud.
With a view to the circumstances described above, an object of the present invention is to provide a cost effective apparatus for preparing solutions in which replacement of an air filter for preventing bacteria or the like from entering into the solution tank is essentially unnecessary, and miniaturization of the entire system and lowering of operation noise are possible.